Balanced Armature
elements or “Sound Powered” elements, as they are called by crystal radio
designers and builders, are the rage for use on Crystal radios. They have
improved performance over the “vintage” style magnetic headphones of the
early 1900’s or any other device made since. Even Baldwins which are the
“Gold Standard” of the vintage headsets do not come close to Sound Powered
headsets for Dxing on a crystal radio. Not always the best for music(some
units), they are much more sensitive than any other type of unit for voice,
which makes them invaluable for picking out that very faint station.

Most Sound Powered headsets and handsets
that are used for this purpose were made for the military during WWII.
These units were made for the Navy ships where it was critical that loss
of power onboard did not knock out communications between “plane spotters”
and the “gunners”, or the bridge and steering or engine rooms. Balanced
Armature or Sound Powered elements were developed that used no outside
power source. The sound from a person’s voice travel to the “Mic” unit,
which generated a small amount of electrical signal which could be picked
up by headset or “ear” unit. No outside power was needed, because it generated
its own power. If a section of the ship was knocked out by a bomb blast
and the wires were destroyed between the spotters and the gunners, a cord
could be rolled out on the deck, and the two units could be reconnected
up in an instant. In this way the ship was not left defenseless for very
long. Two basic types of units were used by the Navy, “Deck Talkers” which
were a headphone and chest mic unit for spotters and gunners, and handsets
which resembled a telephone handset.

The very efficient
design of these units, due to the small signals that were used, is what
makes them perfect for crystal radios. The main problem with these units
is their very low impedance. They must be matched to the radio using matching
transformers to obtain high sensitivity, without “loading” the radio’s
circuit. For more info on matching transformers go to “Matching sound powered
headsets to crystal sets using transformers” link below or to Ben Tongue’s
web page at http://uweb.superlink.net/bhtongue/5hpXform/5hpXform.html.

The “mic” unit can be used
as “ear” piece also. Below are examples of units as they look before being
converted to use on crystal sets. Some are WWII era and some are post-war
era. There are a few units still being made for use today. The fire departments
use sound powered units today because they can be used in areas which may
contain explosive gases without the fear of a spark setting the gases off.
Also they don’t use batteries, which might go dead at the wrong moment.